1. Field of the Invention
The present invention relates to an optical element having a transparent material through which light travels for an optical distance. More specifically, the present invention relates to an optical element having a compensator which deforms the transparent material to reduce changes in the optical distance caused by changes in temperature.
2. Description of the Related Art
Wavelength division multiplexing is used in fiber optic communication systems to transfer a relatively large amount of data at a high speed. More specifically, a plurality of channels, each containing information to be transmitted, is combined into a wavelength division multiplexed light. Each channel is at a different wavelength, and the wavelengths are typically very close to each other along the frequency spectrum.
The wavelength division multiplexed light is then transmitted through a single optical fiber to a receiver. The receiver splits the wavelength division multiplexed light into the individual channels, so that the individual channels can be detected. In this manner, a communication system can transfer a relatively large amount of data over an optical fiber.
FIG. 1(A) is a diagram illustrating a conventional Fabry-Perot etalon interferometer which is typically used to retrieve a single channel from a wavelength division multiplexed light. Referring now to FIG. 1(A), a Fabry-Perot etalon interferometer includes a spacer 10 made of a transparent material and having reflecting films 12 and 14 on opposite sides. The reflectance of reflecting films 12 and 14 are, for example, 95%. Spacer 10 provides stability to the overall structure. However, spacer 10 can typically be omitted, as long as reflecting films 12 and 14 are separated and maintained in parallel with each other.
A collimated light 16 includes lights having different wavelengths and is, for example, a wavelength division multiplexed light. Collimated light 16 enters the Fabry-Perot etalon interferometer and undergoes multiple reflection between reflecting films 12 and 14. Since reflecting film 14 has a reflectance which is less than 100%, the multiple reflection between reflecting films 12 and 14 will cause lights 18 to pass through reflecting film 14.
Moreover, the Fabry-Perot etalon interferometer has strengthening conditions which causes lights 18 to interfere with each other and produce a luminous flux travelling away from reflecting film 14 and including light of a specific wavelength meeting the strengthening conditions. Lights having other wavelengths interfere with each other and thereby weaken each other.
Therefore, the Fabry-Perot etalon interferometer receives collimated light 16 which includes lights having many different wavelengths, and produces a luminous flux which includes only light having a wavelength which meets the strengthening conditions of the Fabry-Perot etalon interferometer. In this manner, a Fabry-Perot etalon interferometer can be used to separate a single channel from a wavelength division multiplexed light.
Since the wavelengths of the channels of a wavelength division multiplexed light are typically very close together, it is important for a Fabry-Perot etalon interferometer to have stable characteristics. The slightest change in characteristics can significantly deteriorate the quality of optical signals transmitted through a fiber optic communication system.
For example, FIG. 1(B) is a graph illustrating a change in transmittance versus wavelength in accordance with a change in temperature, for a Fabry-Perot etalon interferometer. Referring now to FIG. 1 (B), a Fabry-Perot interferometer passes lights with a wavelength meeting a specified condition, and indicated by curves 20. However, if the temperature of the Fabry-Perot etalon interferometer changes, then the Fabry-Perot etalon interferometer will pass lights with a different wavelength, as indicated by curves 22. Therefore, the wavelength of light output from a Fabry-Perot etalon interferometer will undesireably change in accordance with a change in temperature.
A similar problem occurs in a device as illustrated in FIG. 1(C). More specifically, FIG. 1(C) is a diagram illustrating a virtually imaged phased array (VIPA). A VIPA is more fully disclosed in U.S. patent application titled "VIRTUALLY IMAGED PHASED ARRAY AS A WAVELENGTH DEMULTIPLEXER", application Ser. No. 08/685,362, filed Jul. 24, 1996, and which is incorporated herein by reference.
Referring now to FIG. 1(C), a VIPA includes a spacer 24 made of a transparent material. Reflecting films 26 and 27 are provided on opposite sides of spacer 24. The reflectance of reflecting films 26 and 27 are, for example, 100% and 95% respectively. Spacer 24 provides stability to the overall structure. However, spacer 24 can typically be omitted, as long as reflecting films 26 and 27 are separated and maintained in parallel with each other.
Incident light 28 is line-focused to travel through an entrance window 30 and then diverges in spacer 24 so that multiple reflection occurs between reflecting films 26 and 27. Since reflecting film 27 has a reflectance which is less than 100%, the multiple reflection between reflecting films 26 and 27 will cause lights 32 to pass through reflecting film 27. Lights 32 interfere with each other to form collimated light for each wavelength in incident light 28, where each collimated light travels in a different propagating direction than the other collimated lights.
More specifically, the VIPA can receive incident light 28 which includes many wavelengths, and produce a different luminous flux for each wavelength. Each luminous flux has a different angle of propagation from the VIPA and is therefore spatially distinguishable from the other luminous fluxes. Therefore, the VIPA can receive a wavelength division multiplexed light which includes many channels, and produce a spatially distinguishable luminous flux for each channel. A VIPA can be used, for example, as a demultiplexer in an optical fiber communication system which uses wavelength division multiplexing.
A VIPA experiences problems similar to that illustrated in FIG. 1(B) for a Fabry-Perot etalon interferometer. More specifically, a VIPA can undesireably produce a luminous flux having a different wavelength, depending on changes in temperature of the VIPA.
Therefore, the characteristics of a Fabry-Perot etalon interferometer and a VIPA undesireably change as the temperature changes. These characteristic changes are due to the thermal expansion of the spacers used in a Fabry-Perot etalon interferometer and a VIPA. For example, if spacer 10 of the Fabry-Perot etalon interferometer illustrated in FIG. 1(A) is expanded, then the physical distance between reflecting films 12 and 14 is also expanded. Further, the refractive index of spacer 10 changes as the temperature changes. This change in the refractive index causes a change in the optical distance between reflecting films 12 and 14, where the optical distance is a product of the physical distance between reflecting films 12 and 14 and the refractive index of spacer 10.
Similarly, if spacer 24 of the VIPA illustrated in FIG. 1(C) is expanded, then the physical distance between reflecting films 26 and 27 is also expanded. Further, the refractive index of spacer 24 changes as the temperature changes. This change in the refractive index causes a change in the optical distance between reflecting films 26 and 27.
As a result of a change in optical distance corresponding to a change in temperature, the characteristics of a Fabry-Perot etalon interferometer and a VIPA can undesireably change. For example, when a typical glass is used as a spacer in a Fabry-Perot etalon interferometer or VIPA, the thickness of the spacer increases by about 5.times.10.sup.-6 each time the temperature rises by one degree. In addition, the refractive index of the spacer also increases by about 5.times.10.sup.-6 each time the temperature rises by one degree. Assuming that the wavelength of light is 1500 nm and the temperature rises by 10 degrees, the optical distance will change by a factor of 10.sup.-4 and then will result in a wavelength change of 0.15 nm. This is a significant change which can seriously affect the characteristics of a Fabry-Perot etalon interferometer or VIPA.